feat(studio): inline flow editor — YAML highlighting + Run (v0.50.0)

New top-level destination "Editor" (Cmd+5) ships as Studio's
fifth surface. The editor reads + writes flow YAML directly
under ~/.fai/data/flows/ via dart:io — the hub picks up the
changes on the next listFlows / runSavedFlow call.

Layout: two-pane shell. Left (240 px) is the file list; right
flexes to the code pane and an optional results column when
Run produces output. Top toolbar exposes:
  * filename + dirty-mark
  * New flow (scaffolds from a debug.echo template)
  * Save (writes the active file to disk)
  * Run (saves first if dirty, calls
    HubService.runSavedFlow, surfaces typed FlowOutputs in
    a side panel)
  * Refresh

YAML highlighting via flutter_code_editor + the highlight
package's yaml language. Lightweight style map mapping the
five token classes that actually appear in flow YAML
(attr / string / number / comment / subst for the
${{ ... }} template syntax) to FaiTheme colors — keeps the
editor visually consistent with the rest of Studio.

New-flow naming uses a FilteringTextInputFormatter that
restricts the name to [a-z0-9_-]. A "name already exists"
SnackBar surfaces the conflict instead of silently
overwriting.

Bilingual strings shipped (en.arb + de.arb) for every
operator-facing string: toolbar buttons, dialogs, empty
states, file-exists error, run-output header.

New deps:
  * flutter_code_editor ^0.3.5
  * highlight (transitive — pinned as direct so the
    yaml-language import has its declared dependency).

Smoke-test (test/flow_editor_test.dart) pumps the page and
asserts the empty-state + toolbar render without throwing
on hosts that don't have ~/.fai/data/flows yet. The full
file-list + open-on-tap flow needs a writable HOME override
which dart:io's read-only Platform.environment doesn't allow
inside a test isolate — that path lives in the integration
suite as a follow-up.

Version bumps:
  * pubspec.yaml: 0.49.1 → 0.50.0
  * main.dart kStudioVersion: 0.42.0 → 0.50.0 (had drifted
    behind pubspec; brought back into sync as part of this
    bump)

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/l10n/app_localizations_en.dart

@@ -2185,4 +2185,75 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get storeSourceTemporal => 'Temporal';
+
+  @override
+  String get navFlowEditor => 'Editor';
+
+  @override
+  String get flowEditorNew => 'New flow';
+
+  @override
+  String get flowEditorSave => 'Save';
+
+  @override
+  String get flowEditorRun => 'Run';
+
+  @override
+  String get flowEditorRefresh => 'Refresh file list';
+
+  @override
+  String get flowEditorRunOutput => 'RUN OUTPUT';
+
+  @override
+  String get flowEditorEmptyTitle => 'No flow open';
+
+  @override
+  String get flowEditorEmptyBody =>
+      'Pick one from the left, or click New flow to start a fresh one.';
+
+  @override
+  String get flowEditorListEmptyTitle => 'No saved flows';
+
+  @override
+  String get flowEditorListEmptyBody =>
+      'Click New flow to scaffold one from a template.';
+
+  @override
+  String get flowEditorDiscardTitle => 'Discard unsaved changes?';
+
+  @override
+  String get flowEditorDiscardBody =>
+      'Switching files will throw away the edits you haven\'t saved.';
+
+  @override
+  String get flowEditorDiscardKeep => 'Keep editing';
+
+  @override
+  String get flowEditorDiscardThrow => 'Throw away';
+
+  @override
+  String get flowEditorNewDialogTitle => 'New flow';
+
+  @override
+  String get flowEditorNewDialogLabel => 'Flow name';
+
+  @override
+  String get flowEditorNewDialogHelper =>
+      'lowercase letters, digits, hyphen, underscore';
+
+  @override
+  String get flowEditorNewDialogCancel => 'Cancel';
+
+  @override
+  String get flowEditorNewDialogCreate => 'Create';
+
+  @override
+  String flowEditorNewTemplateComment(String name) {
+    return 'Generated by Studio\'s Flow editor. Edit, save, run.';
+  }
+
+  @override
+  String flowEditorAlreadyExists(String name) {
+    return 'A flow named \"$name\" already exists.';
+  }
 }